The ratio of sulfur (S) to selenium (Se) in chalcogenide (e.g., chalcopyrite, kesterite) semiconductors is important for optimal band gap and/or band gap grading. See, for example, T. K. Todorov et al, “High-Efficiency Solar Cell with Earth-Abundant Liquid-Processed Absorber,” Advanced Materials 22, E156-E159 (February 2010). Device-quality material formation typically occurs at temperatures above 450° C. at which S, Se and other key constituents such as zinc (Zn) metal and tin (Sn) chalcogenides are volatile. Furthermore the volatility of Sn species and Sn depletion of the CZTS can strongly depend on the chalcogen type and vapor pressure in the anneal atmosphere (see, for example, A. Redinger et al., “The Consequences of Kesterite Equilibria for Efficient Solar Cells,” J. Am. Chem. Soc. 133, 3320-3323 (February 2011) (hereinafter “Redinger”) and J. J. Scragg et al., “Chemical Insights into the Instability of Cu2ZnSnS4 Films during Annealing,” Chem. Mater., 23(20), pp. 4625-4633 (September 2011) (hereinafter “Scragg”)). Therefore precise real time atmosphere composition control during fabrication is important in order to achieve optimal equilibrium reaction conditions and achieve the targeted profile of volatile species in the material.
With conventional processes, atmosphere compositions of S and Se are often controlled using gases such as hydrogen sulfide (H2S) or hydrogen selenide (H2Se). H2S and H2Se are however highly toxic. There is a considerable expense associated with using and maintaining these gases safely. There are currently no straightforward methods for chalcogenide-dependent Sn vapor control in the anneal atmosphere.
Thus, techniques for effectively regulating S, Se and (optionally Sn) anneal atmosphere composition during device-grade chalcogenide semiconductor production without the use of toxic components such as H2S and H2Se would be desirable.